The assignment of protein backbone and side-chain NMR chemical shifts is the first step towards the characterization of protein structure. The recent introduction of proton detection in combination with fast MAS has opened up novel opportunities for assignment experiments. However, typical 3D sequential-assignment experiments using proton detection under fast MAS lead to signal intensities much smaller than the theoretically expected ones due to the low transfer efficiency of some of the steps. Here, we present a selective 3D experiment for deuterated and (amide) proton back-exchanged proteins where polarization is directly transferred from backbone nitrogen to selected backbone or sidechain carbons. The proposed pulse sequence uses only 1H-15N cross-polarization (CP) transfers, which are, for deuterated proteins, about 30% more efficient than 1H-13C CP transfers, and employs a dipolar version of the INEPT experiment for N-C transfer. By avoiding HN-C (HN stands for amide protons) and C-C CP transfers, we could achieve higher selectivity and increased signal intensities compared to other pulse sequences containing long-range CP transfers. The REDOR transfer is designed with an additional selective π pulse, which enables the selective transfer of the polarization to the desired 13C spins.
Keywords: MAS; Magnetisation transfer; Proteins; Solid-state NMR.